The present invention relates generally to bulk material analysers suitable for the direct on-line analysis of materials such as coal and minerals. It is targeted particularly at direct on-conveyor belt analysis.
A key requirement for direct on-conveyor belt analysis is the ability to measure parameters of interest, such as elemental composition, independently of both horizontal and vertical segregation and independently of changes in belt loading.
Both neutron inelastic scatter (NIS) and thermal neutron capture (TNC) gamma-ray techniques have the advantages of using highly penetrating radiation so that measurements are averaged over a large volume of material on a conveyor belt. They are also capable of the simultaneous quantitative determination of many elements.
Thermal Neutron Capture (TNC) Gamma-Rays
The most widely used technique for on-line bulk material analysis is that based on thermal neutron capture (TNC) gamma-rays (sometimes referred to as the prompt gamma neutron activation analysis (PGNAA) technique). The TNC technique involves bombarding a bulk sample with neutrons from a radioisotope source, usually 252Cf. The 252Cf neutrons (average energy 2.3 MeV) are slowed down to thermal energies (about 0.025 eV) by collisions either in the sample or in an external moderator and then captured by the nuclei of elements present in the sample. The capture process in most cases is accompanied by the immediate release of energetic gamma-rays which are characteristic of the element. In most materials the capture gamma-rays form a complex spectrum of energies, which is capable of interpretation to provide analytical information on the proportion of the various elements present in the sample.
In previous applications of the TNC technique to on-conveyor belt analysis, spatial uniformity is controlled by the use of multiple sources and detectors in transmission geometry together with neutron moderators external to the sample. These external moderators are used to control the thermal neutron flux distribution in the sample to produce a more uniform spatial sensitivity.
In another development, an on-belt analyser has been described which comprises a 14 MeV pulsed neutron generator and a gamma-ray detector located on opposite sides of a conveyor belt. The neutrons are slowed down to thermal energies using heavy metal and polyethylene shields and TNC gamma-ray spectra are measured. However, the problem of spatial sensitivity is not addressed as it is assumed that the material is homogeneous and of constant profile on the belt.
Neutron Inelastic Scatter (NIS) Gamma-Rays
In the NIS technique, fast neutrons undergo direct inelastic scatter reactions with the nuclei of elements in a sample resulting in the production of prompt gamma-rays which are characteristic of the elements present. For NIS to occur the energy of the incident neutrons must be greater than the energy of the gamma-rays. Suitable high-energy neutron sources are 241 Am-Be (average neutron energy about 4.5 MeV), fast neutron generators (neutron energy 2.45 or 14 MeV) and 252Cf (average neutron energy 2.35 MeV). Generally higher energy neutron sources are better suited to NIS applications.
NIS and TNC are in many ways complementary since an element that may not be sensitive to NIS may be highly sensitive to TNC and vice versa. For example, carbon is easily determined using NIS but is only weakly excited in TNC; on the other hand, hydrogen is readily determined using TNC but it produces no NIS gamma-rays. By using a high-energy neutron source NIS is readily combined with TNC techniques to determine the concentration of most of the major elements in a wide range of samples. An example of a successful application combining NIS and TNC is the determination of carbon, hydrogen, ash and chlorine in low rank coal in a by-line geometry.
The NIS technique has been developed for a number of industrial applications on relatively homogeneous materials in sample by-lines. However applications to on conveyor belt analysis have not been attempted partly because of problems with spatial sensitivity of the technique. In a backscatter geometry the effective depth of penetration into the sample is limited. For the example of 4.43 MeV NIS carbon gamma-rays from coal using a 238 Puxe2x80x94Be source, 50% of the measured gamma-rays originate in the first 50 mm of coal and 80% originate in the first 100 mm.
The invention is a bulk material analyser including: A shielded enclosure defining an analysis zone within it and having a passageway through it to allow transport of bulk material through the analysis zone. The shielded enclosure may be made of a material that contains a high hydrogen density, often combined with a material of high neutron capture cross section such as a compound of boron or lithium. The purpose of the shielded enclosure is to provide radiation shielding for personnel. In use, the bulk material is transported though the analysis zone on a conveyor belt or chute which passes along the passageway. At least one neutron source and at least two gamma-ray detectors are disposed within the enclosure to measure gamma-rays produced in the bulk material by both the NIS and TNC processes.
When one neutron source and a gamma-ray detector are arranged in a transmission or backscatter geometry, the spatial response of the gauge will be biased towards either the source side, the centre or the detector side of the sample, depending on the relative attenuation of neutrons and gamma-rays in the sample. The present invention provides a number of alternative methods to overcome this bias using a second detector or second source/detector configuration with a spatial response biased to compensate for the spatial bias of the first configuration. If the two configurations used are both transmission or both backscatter then two sources are required and the source (and detector) of the first configuration will be on the opposite side of the material to the source (and detector) of the second configuration. Variations on the above configurations involve multiple sources and/or detectors which are located across the passageway (perpendicular to the direction of travel) to improve spatial uniformity across the passageway.
Use of the invention can result in significant improvement in spatial uniformity, as a result of the use of two or more of the two possible source-sample-detector configurations, viz., backscatter or transmission configurations.
The neutron sources have sufficient energy to excite NIS gamma-rays from the element of interest; so called xe2x80x9cfast neutron sourcesxe2x80x9d. Neutrons from these sources are also slowed to thermal energies in the sample and surroundings to produce TNC gamma-rays. Suitable sources include radioisotope sources and neutron generators of either continuous or pulsed mode. The neutron-induced gamma-ray measurements may be combined with separate measurements of gamma-ray transmission, thermal neutron flux or fast neutron flux, or any combination of them. NIS measurements may be combined with TNC measurements. Suitable high-energy neutron sources are 241Amxe2x80x94Be, a neutron generator or 252Cf, and suitable detectors are scintillation or solid state detectors such as thallium activated sodium iodide NaI(Tl), bismuth germanate BGO or hyperpure germanium.